drm/i915: update BDW DDI buffer translations

Two BSpec updates changed the recommended values for BDW eDP and DP
DDI buffer translations. Now the signal levels also match the HSW signal
levels, which simplify things a little bit.

It seems some DP sinks don't work properly without voltage level 0 and
pre-emphasis level 3, so this patch may fix some bugs on
panels/monitors that happen on BDW but not on HSW.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>